1. Field of the Invention
The present invention relates to a piezoelectric resonator bending vibrations and a ladder-type filter including such a piezoelectric resonator.
2. Description of the Related Art
As piezoelectric resonators for use in a kHz band or for use in a frequency band of 100 kHz to 1 MHz in particular, piezoelectric resonators utilizing spreading vibrations are well known. In piezoelectric resonators utilizing spreading vibrations, the product of the length Ls of a side and the resonance frequency fr is generally constant, and it is defined by:
As=Lsxc3x97fr
Here, As is a constant (frequency-related constant) and As≅2100 kHzxc2x7mm. For example, in order to obtain resonators having a resonance frequency of fr=455 kHz as in AM radios, the length Ls of the side becomes 4.62 mm.
In recent years, the miniaturization of equipment is more frequently required, and smaller-sized and thinner electronic components are being demanded. Under such circumstances, it is difficult to use electronic components having a length of a side that is 5 mm as described above. Furthermore, in ladder-type filters, the attenuation value is determined by the capacitance ratio between the parallel resonator and the series resonator. That is, in order to obtain a higher attenuation value, the capacitance between terminals of the parallel resonator may be increased and the capacitance between terminals of the series resonator may be decreased. However, when the capacitance between terminals of the parallel resonator is increased, it is inevitable to decrease the thickness of its piezoelectric substrate, and accordingly the mechanical strength is deteriorated and the range of the thickness is therefore very limited.
In consideration of such circumstances, the present inventor has proposed a piezoelectric resonator in which four or more electrode layers and three or more piezoelectric layers are alternately laminated, at least two of the piezoelectric layers are polarized in the thickness direction, and the above-described electrode layers are connected to each other such that, in a portion of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction of the piezoelectric layers and that, in a portion of the other piezoelectric layers, an electric field is generated in a direction that is opposite to the polarization direction of the piezoelectric layers (Japanese Patent Application No. 11-294491).
In this piezoelectric resonator, since the piezoelectric layer in which the polarization direction and the direction of electric field are the same contracts in the planar direction and the piezoelectric layer in which the polarization direction and the direction of electric field are opposite to each other expands, bending vibration is generated as an entire piezoelectric resonator. In such a piezoelectric resonator utilizing bending vibrations, the dimensions thereof can be more reduced, compared with a piezoelectric resonator utilizing spreading vibrations, when both have the same resonance frequency. Moreover, since four or more electrode layers are provided, capacitance can be individually generated between electrode layers, and the total capacitance between terminals can be increased. Moreover, the piezoelectric layers are laminated, and accordingly even if each piezoelectric layer is thin, there is an advantage that a sufficient mechanical strength can be achieved.
However, in the piezoelectric resonator having the above-described construction, since an input electrode and an output electrode are separately disposed on both top and bottom main surfaces, when the piezoelectric resonator is mounted on a board, or substrate, after the electrode on one main surface has been connected to an electrode of the board, the electrode on the other main surface must be connected to another electrode on the board by wire bonding or by using terminals, etc., and accordingly it was difficult to establish connections. That is, the above-described piezoelectric resonator was not constructed in such a way that electrical characteristics can be taken out from either of the top and bottom main surfaces, and it was difficult to surface mount the piezoelectric resonator.
In order to overcome the problems described above, preferred embodiments of the present invention provide a very small-sized piezoelectric resonator in which electric characteristics can be taken out from either of the top and bottom main surfaces and which is appropriate for surface mounting, and to provide a ladder-type filter including such a piezoelectric resonator.
According to a first preferred embodiment of the present invention, a piezoelectric resonator utilizing bending vibrations, includes three or more electrode layers and two or more piezoelectric layers that are alternately laminated, first and second end surface electrodes provided on the end surfaces of the laminated piezoelectric layers, recessed portions for insulating the electrode layer, which is not connected to the first or second end surface electrodes, from the first or second end surface electrodes, the recessed portions provided in the periphery portion of the electrode layer not connected to the first or second end surface electrodes, insulation layers provided on the surface of the outermost electrode layers, and first and second external electrodes, which are connected to the first and second end surface electrodes respectively, provided on the outer surface of the insulation layers. At least a first and second of the piezoelectric layers are polarized in the thickness direction, and the first and second end surface electrodes and the electrode layers are connected such that, in at least the first of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction in at least the first of the piezoelectric layers and that, in at least the second of the piezoelectric layers, an electric field is generated to be opposite to the polarization direction in the piezoelectric layer.
According to another preferred embodiment of the present invention, a piezoelectric resonator utilizing bending vibrations, includes three or more electrode layers and two or more piezoelectric layers alternately laminated, first and second end surface electrodes provided on the end surfaces of the laminated piezoelectric layers, a recessed portion, which is insulated from the first end surface electrode, provided in the periphery portion of one outermost layer of the electrode layers, and a first external electrode, which is connected to the first end surface electrode, provided in the recessed portion, and a recessed portion, which is insulated from the second end surface electrode, provided in the periphery portion of the other outermost electrode layer, and a second external electrode, which is connected to the second end surface electrode, provided in the recessed portion. Then, at least two of the piezoelectric layers are polarized in the thickness direction, and the first and second end surface electrodes and the electrode layers are connected such that, in at least a first one of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction in the first of the piezoelectric layers and that, in at least a second one of the piezoelectric layers, an electric field is generated to be opposite to the polarization direction in the second of the piezoelectric layers.
Regarding the first preferred embodiment of the invention, since a piezoelectric layer in which the polarization direction and the direction of electric field are the same contracts in the planar direction and a piezoelectric layer in which the polarization direction and the direction of electric field are opposite to each other expands, bending vibration is generated as an entire piezoelectric resonator. In such a piezoelectric resonator utilizing bending vibrations, the dimensions thereof are greatly reduced, compared with a piezoelectric resonator utilizing spreading vibrations, when the resonance frequency is the same. For example, in order to obtain a resonator having a resonance frequency of fr=455 Hz, the resonator utilizing spreading vibrations has a side with a length of about 4.62 mm, but, in a resonator utilizing bending vibrations, the length of a side can be made to be about 1.2 mm, when the thickness of the resonator is about 0.2 mm. Furthermore, the first and second end surface electrodes and the electrode layers are connected, the insulation layers are provided on the surface of the outermost electrode layers, and the first and second external electrodes, which are connected to the first and second end surface electrodes, are provided on the outer surface of the insulation layers, and accordingly electrical characteristics can be taken out from either of the main surfaces of the piezoelectric resonator. That is, the piezoelectric resonator can be easily constructed as a surface mounting type component.
When the electrode layers are connected to each other by using the end surface electrodes, it is required to alternately connect the electrode layers to one end surface electrode and make the electrode layers alternately insulated from the end surface electrode. In order to make the electrode layers insulated from the end surface electrodes, insulators may be provided between the end surface electrodes and the electrode layers, but, as a result, this forces the insulators to be provided in the very narrow spaces, the manufacturing process becomes complicated, and the cost increases. Then, by forming the recessed portions in advance when the electrode layers are patterned on the piezoelectric layers, the electrode layers can be easily insulated from the end surface electrodes, and the manufacturing process can be simplified.
According to various preferred embodiments of the present invention, it is desirable that the piezoelectric layers and the insulation layers have a substantially square-form, that the first and second end surface electrodes are disposed substantially in the middle of the four sides of the piezoelectric layers, and that the first and second external electrodes are disposed substantially in the middle of the insulation layers. In the case of a substantially rectangular piezoelectric resonator utilizing bending vibrations, longer side and shorter side vibrations exist, but when the shape becomes close to a substantially square configuration, the frequencies of the shorter sides and longer sides approach each other, and when the shape finally comes to be substantially square, both frequencies coincide with each other to produce a very strong vibration. In the bending vibration, the nodal points are located in the vicinity of the circumference inscribed in the four sides. Accordingly, when the end surface electrodes and the external electrodes are disposed in the vicinity of the contacts between the nodal points and the four sides, that is, substantially in the middle of the four sides and the external electrodes are mounted on a board or other substrate, the bending vibration is not damped when mounted and excellent characteristics are achieved. In accordance with various preferred embodiments of the present invention, by forming the external electrodes in the vicinity of the nodal points of bending vibrations, excellent characteristics are achieved.
According to preferred embodiments of the present invention, the piezoelectric layers may be substantially square-shaped, and the first and second end surface electrodes may be disposed at the corner portions of the piezoelectric layers. When the end surface electrodes are provided at the corner portions, the recessed portions of the outside and inner electrode layers can be provided at the corner portions. In the piezoelectric resonator utilizing bending vibrations, the electrical characteristic (xcex94f) is affected by the shape of the electrodes, and then least affected at the corner portions. Therefore, by forming the end surface electrodes at the corner portions, a piezoelectric resonator in which the electrical characteristic is not affected can be obtained.
According to preferred embodiments of the present invention, the recessed portions of the electrode layers may be formed to be belt-shaped along the periphery portion of the electrode layers. In the case in which the belt-shaped recessed portions are provided, when the electrode layers are formed, it is enough to align the corner portions and the displacement in length and width directions can be neglected. Therefore, the electrode layers can be easily positioned when the piezoelectric layers are laminated.
According to various preferred embodiments of the present invention, the piezoelectric layers may be formed to be substantially square-shaped, and the first and second end surface electrodes may be provided on the opposing sides of the piezoelectric layers. In this case, the external electrodes for input and output purposes are provided on the opposing sides, and it is enough to provide electrode pads at two locations when mounted on a circuit board or other substrate. In the case in which the first and second end surface electrodes are provided on the adjacent sides of the piezoelectric layers, it means that the external electrodes for input and output purposes are provided on the adjacent sides, and then it is required to provide electrode pads at four locations on a circuit board.
According to preferred embodiments of the present invention, the outermost electrode layer is commonly used as one external electrode and the other external electrode is disposed in its recessed portion, and then without providing any insulation layers, the same operation as in the first preferred embodiment of the present invention can be realized, that is, electrical characteristics can be taken out from either of the main surfaces.
According to various preferred embodiments of the present invention, it is desirable that concave grooves, which are continuous in the thickness direction, are provided in the end surfaces of the laminated piezoelectric layers, and that the end surface electrodes are constructed by using the electrodes disposed on the inner surface of the concave grooves. The concave grooves and their inner surface electrodes can be easily arranged such that through-holes passing through the piezoelectric layers laminated in the state of a mother board are provided, that an electrode film is formed on the inner surfaces thereof, and that the laminated piezoelectric layers are cut at the location of the through-holes. Furthermore, since the inner surface electrodes can be formed at the same time when electrode layers are formed on the surface of piezoelectric layers, a particular process for forming the end surface electrodes is not necessary, and accordingly the manufacturing process is greatly simplified.
According to preferred embodiments of the present invention, a very small-sized ladder-type filter having excellent filter characteristics can be obtained by providing a piezoelectric resonator utilizing bending vibrations to define a series resonator and a parallel resonator in the ladder-type filter. In the case of ladder-type filters, a plurality of series resonators and a plurality of parallel resonators are often used in order to obtain a high attenuation value. When a very small-sized piezoelectric resonator according to preferred embodiments of the present invention is used as a series resonator and a parallel resonator, a very small-sized ladder-type filter as a whole can be realized.
According to various preferred embodiments of the present invention, it is desirable to construct the parallel resonator by using a piezoelectric resonator having more piezoelectric layers than a piezoelectric resonator in the series resonator. The attenuation value in a ladder-type filter is determined by the capacitance ratio between the parallel resonator and the series resonator. In the piezoelectric resonator according to preferred embodiments of the present invention, the capacitance between terminals can be arbitrarily increased by increasing the number of piezoelectric layers. Then, when the parallel resonator is constructed by using a piezoelectric resonator having more piezoelectric layers, the capacitance ratio between the parallel resonator and the series resonator can be unrestrictedly increased, and accordingly the attenuation value of a filter can be increased without restraint.
According to preferred embodiments of the present invention, it is desirable that the series resonator and the parallel resonator utilizing bending vibrations are mounted in one plane on a board on which input electrodes, output electrodes, grounding electrodes, and connection electrodes are patterned, and that a cap is fixed on the board so as to cover the series resonator and the parallel resonator. Since electrical characteristics can be taken out from either of the top and bottom main surfaces, the piezoelectric resonator according to preferred embodiments of the present invention can be arranged in one plane on a board, and electrical connections to the board can be simplified.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.